Electrical conductors play fundamental roles in modem civilization, and their improvements are of substantial importance and utility. Ongoing problems in electrical systems include (a) the need for materials exhibiting low electrical resistivity so as to minimize loss of the energy and (b) the need for new conducting elements of nanoscale size to be suitable for future, small scale microelectronics. Any significant increase in density of electronic elements raises the problem of overheating. Minimizing overheating can require advances both problems (a) and (b).
Any electrically neutral substance, including electrical conductors have equal amounts of positive and negative charges and, in general, a conductor's properties can depend on the physical state of both kinds of charges. All good conductors can be considered in few classes considered below from the viewpoint of the crystallinity of their charges. For example, solid metals fall to the class of partially crystalline conductors, because their positive ions form crystalline lattice and their negative charges are in a disordered state which can be described as a stable free electron gas.
Prior art conductors can be considered to be members of one or more different classes, depending upon certain characteristics.
Class I: Partially Crystalline Solid Conductors
All materials in the class of partially crystalline solid conductors are solid inorganic conductors consisting of an immovable crystalline lattice of positive ions and a disordered gas of conducting free electrons.
Solid metals and alloys (e.g., of copper, aluminum and like materials) made of inorganic elements are presently among the most widely used conductors, because at ambient temperatures, the best of them have conductivity≅106 Sim/cm, and wires made of these metals exhibit continuous current carrying capacity of about 103 A/cm2. The ionic crystalline lattice of these materials has no chemical bonds, and is stable only because of so-called metallic binding. Free electrons in metals obey Fermi-statistics and have a concentration of about 1023 cm3. These conditions lead to high kinetic energy of the electrons and to relatively weak electron-electron correlations.
A property of many metals, discovered in 1911 by Kamerlingh-Onnes, is that below some critical temperature Tc they suddenly lose their resistivity and become superconductors (SC), i.e. materials having extremely high conductivity. Later it was discovered by Meisner that the transition of bulk metal from normal (i.e. resistive) to the SC state is accompanied by the sudden appearance of extremely strong diamagnetism that expels the magnetic field from the SC. Basically, a SC appears as a consequence of specific electron correlations, and usually, a higher Tc corresponds to a stronger correlation. Unfortunately, metal SC are of limited technical utility because their weak electron-electron correlation may result only in rather low Tc, of about 20K or less.
The creation of metallic nanowires having diameters less than 100 nm was described in U.S. Pat. No. 4,325,795 (1982) by R. Bourgoin. In accordance with this teaching, nanowires having some properties similar to SC at ambient temperatures should be prepared within a liquid dielectric polymer medium, namely epoxy resin, with the use of a very fine powder of conventional metal, namely bismuth, constituting not less than 10 vol. % in this mixture. The organic polymer medium does not contribute any charges participating in the formation of nanowires. The patent does not describe how only the decrease of the diameter of the bismuth wire results in superconductivity at ambient temperatures. Bulk bismuth is well known to have Tc=5 K, and even substantial reduction of its size (for instance, in very thin films) increases Tc up to only about 8 K. Nonetheless, direct participation of large amounts of conventional metals to conductivity places this metal-organic composite material in the same conducting class as pure metals.
Class II: Completely Crystalline Conductors
The class of completely crystalline conductors includes both inorganic materials and organic molecular materials. In both cases positive and negative charges are well localized and form a common crystalline lattice.
New ceramics discovered by J. Bednorz and K. Muller (Z. Phys. B, Vol. 64, p. 188 (1986)) are examples of completely crystalline inorganic conductors. These crystals have no molecular structure, and the chemical elements are bound partially by covalent bonds and partially by ionic bonds. Sizes of both positive ions and negative ions as well as the distance between them do not usually exceed 0.2 nm. Electrons are not free in such crystals because they are strongly trapped by those elements, thereby forming negative ions. Broken stoichiometry is required, or alternatively, other elements have to be introduced to play the role ofdoping agent creating ‘holes’ in an electron spectrum. At high temperatures, these holes can move by a hopping mechanism, resulting semiconductor-type conductivity. Actually that means the formation of “defects” in a negatively charged crystal. The low concentration of such defects (between 1021 and 1022 cm−3) causes only weak disturbance of crystalline state of the negative charges and allows charge carriers to correlate much better. Thus, ceramics may become highly conductive materials with relatively high Tc of about 100K. Their superconductivity exhibits reduced dimensionality in contrast to metals, which have three-dimensional superconductivity. However wide application of superconductive ceramics has not been possible, mainly because of at one or more of the following reasons:
In general, superconductivity is a highly complicated physical phenomenon. Since first discovery, it took half-century to develop BCS theory explaining metallic SC (J. Bardeen, L. Cooper, J. Schrieffer, Phys.Rev. V. 108, p.1175 (1957)). According to the BCS theory, superconductivity may occur only from electron pairing mediated by a mechanism of energetic exchange within an electron couple, becoming a new particle which doesn't obey Fermi-statistics. For instance, in metals and ceramics, delocalized superconducting pairs have charge −2e and spin S=0, and are condensed on the same energetic level. As a result of our invention, BCS theory exploiting only electron-phonon pairing mechanisms is not universal, and fails to describe so-called “high temperature SC” in new ceramics, as well as some other related effects. For instance, on the basis of BCS, it was always thought that any SC phenomenon is incompatible with ferromagnetism. Nonetheless, the first inorganic materials combining SC with ferromagnetism at about liquid helium temperatures were discovered recently by America scientists and others (see, for instance S. S. Saxena et al., Nature, v. 406, p.587 (2000); C. Pfleiderer et al., Nature, v.412, p. 58(2001); D. Aoki et al., Nature, v.413, p.613(2001)). This discovery demonstrates that mechanisms other than BCS mechanisms may also exist, and some conventional definitions of SC, including those based on the BSC Meisner effect, may not be widely applicable.
In 1980's some solid salts made of organic molecules with molecular weight about few hundreds a.m.u. were discovered to become SC at very high pressure about thousands MPa and Tc less than 12K. These salts form well defined molecular crystals whose lattice is built of both positively and negatively charged chemical groups having approximate size about 0.2–0.3 nm and separated by the same small distances. That is a reason why Coulomb forces cause strong binding of electrons within such lattice. Similar to ceramics conducting “defects” can be induced by appropriate doping agents as well as by high pressure causing strong deformation of both positive and negative crystalline sub-lattices. There is presently no full theoretic understanding of intrinsic mechanism of a superconductivity in these salts.
Class III: Amorphous Conductors
Amorphous conductors include the following sub-classes:
II-1. The first sub-class includes liquid conductors in which both positive and negative charges contribute in conductivity. Due to random distribution of the charge carriers, neither form crystalline lattices.
(a) The conductivity of the group of liquid metals (e.g., mercury) and some alloys made of inorganic elements and containing no molecules, is mainly due to a negatively charged gas of free electrons. Slowly moving positive charges are well localized metallic ions having typical radii of less than about 0.2 nm. At ambient temperatures, the conductivity of these liquids is about two orders of magnitude less than that of best solid metals like copper;
(b) Liquid molecular conductors are represented, for example, by melted salts and electrolytically dissociated organic or inorganic molecules dissolved in a solvent. Conductors in this class maybe composed of elements, molecular groups, or both, and exhibit conductivity even less than that of liquid metals. In contrast to liquid metals, moving negative charges are represented in this group by disordered negative ions in which well localized electrons are captured within anionic atoms, molecules or anion chemical groups having relatively high electron affinity and characteristic sizes of 0.5 nm or less.
III-2. The second sub-class includes solid conductors in which no ions participate in conductivity and neither positive nor negative charges form crystalline lattice;
(c) Some amorphous solid metals made of inorganic elements have random distribution of positive ions, and conduct only due to the presence of disordered free electrons. In contrast to liquid conductors, a fixed state of localized positive ions prevents their direct participation in conductivity. No information has been reported that such conductors can exhibit higher ambient temperature conductivity than that of copper or gold;
(d) A well known example of a solid organic macromolecular conductor is polyacetylene (PA), which has alternating single and double bonds in a long molecular chain. Such a “polyconjugated” system results in the creation of free electrons delocalized along the whole macromolecular chain. If heavily doped with iodine, the PA and other conjugated polymers can show relatively high conductivities from 103 to 105 Sim/cm. Although PA has high crystallinity there is no indication that its ionic components can form a crystalline lattice or can directly participate in charge transfer.
(e) Another solid conducting material made of non conjugated, non-doped macromolecules is based on a so-called “superpolaron” quantum design disclosed in U.S. Pat. No 5,777,292; L. Grigorov et al., inventors. This material contains local conducting structures having diameters of about 1000 nm, typical lengths of about 20 microns, and exhibit conductivity at ambient temperatures more than 106 Sim/cm. These structures were independently confirmed by V. Arkhangorodski et al., JETP Lett. V. 51, p. 67 (1990). The physical state of electrons in superpolaron structures was fully predicted by quantum mechanical methods (L. Grigorov, Sov. Tech. Phys. Lett. Vol. 17, p. 368 (1991)). This article described a superpolaron as a thread-like system where all free electrons exist in a cylindrical potential well formed by oriented polar groups of surrounding macromolecules. Such orientation takes place only if the medium has a high dielectric constant and, due to elasticity of macromolecules, it saturates within rather large radius, wherein R0≧2 nm whose value explicitly defines both the geometry and main electronic properties of the superpolaron. For instance, within the thread, the equilibrium mean distance between free electrons cannot be less than 2R0≧4 nm resulting in local electron concentration≈2·1019 cm−3 or less. The diameter Dof a superpolaron's macromolecular shell containing randomly distributed macroions is be D>>2R0; the local concentration of positive ions cannot exceed 1018 cm−3.
All superpolaron's electrons can be described as stationary quasi-one-dimensional plane waves concentrated near the well's axis and are delocalized along its length. These charge carriers are typical for normal metal Fermi-particles having charge −e and spin S=½. Their conductivity maybe a natural consequence of reduced dimensionality. This theory states no specific superconducting electron correlation or pairing mechanism. At the same time, experimentally observed slight ferromagnetism demonstrated the theoretic statement that electron exchange interactions as well as the presence of positive macromolecular ions partially decreasing free electron repulsion contributes to form such systems.
However, systems based on quasi-one-dimensional quantum designs having on low density of free electrons has an intrinsic instability due to a strong tendency of localization of free electrons causing them to lose mobility. This tendency is enhanced in a quasi-liquid macromolecular medium where the field of localized electrons may easily reorient dipole groups. If that happens, then delocalized electron threads becomes suddenly broken. That's why long-term stabilization of ultrahigh conductivity based on superpolarons requires the material to be solidified. Nonetheless, even that solid state should be considered as as metastable, like that of tempered steel.
Practical use of prior art materials is inherently limited by both the physical state of delocalized electrons within superpolarons and by several difficulties of their creation.
(a) The choice of material precursors is limited to amorphous macromolecules only.
(b) Producing of macroions and free electrons is limited due to long UV-irradiation causing molecular decomposition.
(c) Bulk production seems to be impossible in frameworks of the prior art discussed, due to the fact that ions and free electrons can be generated only on a solid state surface covered with very thin film of macromolecular substance.
(d) Slow diffusion of macromolecular ions from the surface limits mean electron concentration to a low value (3·1017 cm−3) and thus prohibits the formation of any other, more efficient quantum structures.
(e) Due to the metastable nature of superpolarons, the ready solidified material may not be further plasticized with solvents or permitted to undergo any substantial mechanical deformation such as those necessary for manufacturing many kinds of real products. All of these disadvantages reflect the fact that neither the chemical nature of macromolecular ions nor the intimate mechanisms of their creation have been understood. Thus, there is limited ability to control all these processes and their consequences.
Class IV: Non Conventional Conductors Having Lowered Dimensionality
Non-conventional conductors having lowered dimensionality have been described.
(a) For example, W. Little, professor of physics at Stanford University, was the first one who considered easily polarized chemical groups as potential mediators of electron pairing, and proposed rather unusual quantum theory of ambient quasi-one-dimensional superconductivity in a special class of conjugated polymers (Phys. Rev. Vol. 134, p. A1416, (1964)). No practical realization of this idea has been reported yet.
(b) Another unusual quantum conductor, a so-called Wigner crystal, was predicted by E. Wigner in 1934. Wigner crystals may appear at decreased concentration of free electrons about or below 1020 cm−3, and is usually considered to be a self-localized cubic electron lattice immersed within uniformly distributed positive ionic “jelly.” There is no overlapping of electron wave functions related to neighboring nodes of the lattice, and the crystal is formed only because of an energetic gain due to electron localization. It is thought that above some concentration threshold of about 1021 cm−3, conventional Wigner crystals undergo phase transition called “electron crystal melting” and behaves like a usual metal. For many decades it traditionally seemed that classic Wigner crystals, based on s-type electron wave functions, corresponded to the lowest energetic state of such electronically diluted system. In other words, it was considered as a ground state (D. Pines, in: Elementary Excitations in Solids, New York-Amsterdam, 1963). In a perfectly uniform positively charged jelly, a Wigner crystal should not be bound with a matter and, therefore, represents an ideal conductor able to move as a whole. However, the charge of real positive ions can never be absolutely uniform. That is why Wigner crystals recently observed in 2-dimensional semiconductor heterojunctions are good insulators pinned to a disordered ionic sub-lattice at low applied voltage (V. Goldmann et al., Phys. Rev. Lett., Vol. 65, p. 2189 (1990)). A quantitatively moving classic Wigner crystal and its low voltage pinning effect were computed and described by physicists of University California, Davis and University of Michigan (C. Reichhardt et al., arXiv: Cond-Mat/0007376, Jul. 24, 2000).
(c) The traditional viewpoint was altered in 1998 by E. Fradkin and S. Kivelson, professors of physics at University Illinois and University of California Los Angeles. They showed theoretically that in a strong magnetic field, a diluted two-dimensional electron gas is able to form a new quantum electron liquid crystal taking place on phase diagram in between normal metal and a pinned Wigner crystal (Cond-Mat/981015, Oct. 13, 1998). The repulsion of non-bound electrons can result in a specific correlation combining coherent movement with crystalline order. Moreover, in 1999 French theorists showed that a two-dimensional quantum crystal of spinless fermions (i.e. electron spin of S=0) can even exhibit persistent current, reflecting very high conductivity (G. Benenti et al., Cond-Mat/9905028, May 3, 1999). Both these articles described crystallized electrons as individual particles having the usual charge, −e. However, both papers are purely quantum theory manifesting a great potential but no materials based upon those considerations has been produced.